Create union type annotation from dynamically generated array in Typescript

Question

Giving a generated array from a reduce, how is it possible to create a union type from this array?

import someFancyObject from 'some/modules';
const generatedArray = someFancyObject.reduce((acc, val, i) => {...}, []);

// generatedArray would be like: ['a', 'b', 'c']

type UnionFromArray = ???

// UnionFromArray would be like 'a' | 'b' | 'c'

Answer 1

You can either infer an array from function argument or declare constant (immutable) array:

const arr = ['a', 'b', 'c'] as const
type UnionFromArray = (typeof arr)[number] // "a" | "b" | "c"

// OR

const unionFromArray = <
  T extends string,
  Arr extends T[]
>(arr: [...Arr]): Arr[number] => null as any

const result = unionFromArray(['a', 'b', 'c']) // // "a" | "b" | "c"